Ls process for the manufacture of sebacic acid diesters of higher alcoho

ABSTRACT

A PROCESS FOR THE MANUFACTURE OF SEBACIC ACID DIESTERS OF HIGHER ALCOHOLS BY ELECTROCHEMICAL CONDENSATION OF THE CORRESPONDING ADIPIC ACID MONOESTERS IN METHANOLIC SOLUTION, WHEREIN THE FLOW OF CURRENT THROUGH THE ELECTROLYTIC CELL IS INTERMITTENTLY STOPPED AND THE ELECTRODES ARE SHORTCIRCUITED.

United States Patent 3,756,928 PROCESS FOR THE MANUFACTURE OF SEBACIC ACID DIESTERS OF HIGHER ALCOHOLS Wolfgang Eisele, Heinz N ohe, and Hubert Suter, Ludwigshafen, Germany, assignors to Badische Anilin- & Soda- Fabrik Aktiengesellschaft, Ludwigshafen (Rhine), Germany No Drawing. Filed Aug. 3, 1971, Ser. No. 168,733 Claims priority, application Germany, Aug. 12, 1970,

Int. Cl. C07b 29/06; C07c 69/50, 67/00 US. Cl. 204-59 R Claims ABSTRACT OF THE DISCLOSURE A process for the manufacture of sebacic acid diesters of higher alcohols by electrochemical condensation of the corresponding adipic acid monoesters in methanolic solution, wherein the flow of current through the electrolytic cell is intermittently stopped and the electrodes are shortcircuited.

This invention relates to a new process for the electrochemical production of sebacic acid diesters of higher alcohols.

It is known that sebacic acid diesters may be produced electrochemically by anodic condensation of adipic acid monoesters (Kolbe reaction). In this known process monomethyl adipate is usually used as starting material. To achieve good yields, the anodic condensation is carried out in methanolic solution at an anodic current density of, say, approximately 25 amps/drnfi, part of the monoester having been converted to a salt, for example by adding sodium carbonate or sodium methoxide, in order to provide adequate conductivity.

'If it is desired to use adipic acid monoesters of higher alcohols as starting materials, considerable difficulties are encountered due to the fact that a polymeric coating is deposited on the anode. One result of this is that the cell potential rises until the passage of current and consequently the reaction comes to a stop.

An attempt has been made to counteract the undesirable formation of deposits on the anode by stopping the flow of current through the cell at intervals and adding certain ethers and acetic acid. These measures prevent a sharp rise in potential during relatively short runs but are ineffective in continuous operation, in which a considerable rise in potential may be observed after only about 24 hours. This leads to an increase in the current consumption from for example 3 kWh/kg. of di-2-ethylhexyl sebacate to more than double this value, when the adipic half ester of 2- ethyl-hexanol is used as starting material.

Since sebacic acid esters of higher alcohols are very important plasticizers and synthetic lubricants, it is particularly desirable to produce these esters, for example di-2- ethylhexyl sebacate (which may be obtained by condensation of monomethyl adipate followed by transesterification), from the adipic monoesters of higher alcohols direct using the Kolbe reaction.

We have now found that such a synthesis is made possible by the process of the invention. According to our new process, sebacic acid diesters of alcohols of from 4 to 12 carbon atoms may be produced by electrochemical condensation of the corresponding adipic acid monoesters in methanolic solution in a particularly advantageous manner and without the above drawbacks, when the flow of electric current through the electrolytic cell is intermittently stopped and the electrodes are short-circuited.

After a period of current flow of from 1 to 30 minutes and more particularly of from 2 to 20 minutes, the flow of current through the electrolytic cell is stopped for advantageously from 2 to 60 seconds and preferably from 5 to seconds. During these current-off periods the electrodes are short-circuited.

These measures of switching off the current and shortcircuiting the electrodes are highly beneficial in long continuous operation. They prevent the occurrence of undesirable deposits on the anode so that the resistance of the electrolytic cell and the conversion of matter are not impaired. These measures are conveniently effected by arranging for an electric time switch to switch oil the rectifier and short-circuit the electrolytic cell automatically at specific time intervals.

Suitable adipic acid monoesters are monoesters of adipic acid with primary or secondary straight-chain or branchedchain aliphatic alcohols of from 4 to 12 carbon atoms and preferably from 6 to 10 carbons atoms, for example monobutyl, mono-isobutyl, mono-2-butyl, monopentyl, monohexyl, monocyclohexyl, mono-octyl, mono-Z-methylhexyl- 5, mono-2-ethylhexyl, monodecyl and monododecyl adipates. Of particular commercial importance is the use of mono-2-ethylhexyl adipate.

-In our new process, the adipic acid monoesters are used in 20-50% and preferably -40% w./w. methanol solutions. The acidic solutions are partially neutralized to ensure adequate conductivity of the electrolyte. This is effected by adding basifying compounds such as sodium bicarbonate, sodium carbonate or sodium methoxide. That molar proportion of the adipic acid monoester which is neutralized is referred to as the degree of neutralization. It is from 1 to 30% and preferably from 2 to 15% molar. It is advantageous to add organic solvents, preferably ketones, such as methylethyl ketone or cyclohexanone, to the methanolic electrolyte in a concentration of from 5 to 20% based on the total weight of the electrolyte. The addition of acetic acid in a proportion of from 0.5 to 2% is also beneficial.

The condensation is carried out at current densities of, say, from 5 to 50 amps/dm. and preferably from 10 to 30 amp.s/dm. The resulting cell potentials are, acording to conditions, from 10 to 30 volts but generally from 11 to 19 volts. The best results are obtained at high current densities and low degrees of neutralization.

The temperature of the reaction mixture is usually maintained at from 20 to 65 C. and preferably from 30 to 55 C. The upper temperature limit is set by the boiling point of methanol. Particularly suitable anodes are smooth platinum anodes. Alternative anode materials, however,

are platinum-rhodium, platinum-ruthenium, platinumiridium, gold, gold-platinum alloys such as :10 gold/ platinum, platinized titanium or tantalum, gold-plated titanium or tantalum, and noble metal oxides, Requirements are less exacting as regards the cathode material, which may be platinum, stainless steel, nickel or titanium.

The reaction may be conveniently carried out in a cell such as that described in Belgian Pat. 723,694, which has vibrating pairs of electrodes which are permeable to liquids. Alternatively, the reaction may be carried out in cells which vibrate and have one permeable and one impermeable electrode or which have two impermeable electrodes and do not vibrate.

Particularly advantageous results are obtained when the reaction is carried out in an electrolytic cell such as that described in German published application DOS 2,039,590. Such a cell consists of bipolar electrode plates assembled in the manner of a filter press, the electrolyte being circulated by a pump from a buffering vessel through the cell chamber, a cooler and a degassing apparatus and so back to the buffering vessel. The degree of conversion (consumption of free carboxyl groups) may be determined by titration or by measuring the pH of the electrolyte. In con tinuous operation, fresh electrolyte is metered from a storage vessel and reacted electrolyte is discharged by overflow means. The feed of fresh electrolyte is conveniently controlled by the pH meter.

In the following examples the parts and percentages are by weight.

EXAMPLE 1 Batchwise operation A solution of 30 parts of mono2-ethy1hexyl adipate in 70 parts of a mixture of 90 parts of methanol, 10 parts of methyl ketone and 0.5 part of acetic acid is adjusted to a degree of neutralization of 10% with sodium methoxide and pumped through a cell such as is described in German published application DOS 2,039,590, in which the gap between the bipolar electrodes is 0.5 mm., the current density being 20 amps/dmF, the temperature being from 40 to 45 C. and the linear rate of fiow being 50 cm./s. The anode and cathode areas are each 2 dm. The flow of current through the electrolytic cell is stopped for 30 seconds at intervals of 5 minutes. During these current-off periods the electrodes are short-circuited. Electrolysis takes place at a cell potential of 15 volts.

Electrolysis is continued until about 95% of the monoester has reacted. The solvents are then evaporated off and the residue is washed with 20% sodium hydroxide solution and then with water and freed from by-products by steam distillation at 20 mm. of Hg and 90 C. The purity of the ester obtained as residue is checked by gas chromatography. There is obtained di-(Z-ethylhexyl) sebacate in a yield of 85% and at a current efiiciency of 65%. The energy consumed is 3 kWh/kg. of di-(2-ethylhexyl) sebacate.

If no ketone is added, the potential rises to about 30 volts and the energy consumption is doubled. This also occurs if the measures of switching ofi the current and short-circuiting the electrodes are omitted.

EXAMPLE 2 Continuous operation A solution of 30 parts of mono-(Z-ethylhexyl) adipate in 70 parts of a mixture of 90 parts of methanol, parts of methylethyl ketone and 0.5 part of acetic acid is adjusted to a degree of neutralization of 10% with sodium methoxide and is circulated through the cell described in Example 1. Electrolysis is effected at a current density of 20 amps/dmF, a temperature of from 40 to 45 C. and a potential of from 12 to 14 volts. The feed of fresh electrolyte is adjusted so that the electrolyte circulated through the cell is reacted to the extent of from 80 to 90%. The reacted electrolyte is discharged continuously. The electrolysis current is switched off for periods of 15 seconds at intervals of 10 minutes, the electrodes being short-circuited during the current-off periods. The discharged electrolyte is worked up as described in Example 1. There is obtained di-(Z-ethylhexyl) sebacate in a yield of 80% and at a current efficiency of 60%.

EXAMPLE 3 Continuous operation The electrolyte employed in Example 2 is used except that the 10 parts of methylethyl ketone are replaced by 10 parts of cyclohexanone. The same cell is used but at a current density of 15 amps/dm. a temperature of to C. and a potential of from 10 to 12 volts. The degree of conversion of the electrolyte is from to At intervals of 6 minutes, the electrolysis current is switched off and the electrodes are short-circuited for a period of 15 seconds. The discharged material is worked up as described in Example 1 to give di-(Z-ethylhexyl) sebacate in a yield of 80% and at a current efiiciency of 60%.

We claim:

1. A process for the manufacture of diesters of sebacic acid with alcohols of from 4 to 12 carbon atoms by electrochemical condensation of monoesters of adipic acid with alcohols of from 4 to 12 carbon atoms in methanol solution wherein the flow of current through the electrolytic cell is intermittently stopped for periods of from 2 to 60 seconds at intervals of from 1 to 30 minutes and the electrodes are short-circuited during the current-off periods.

2. A process as claimed in claim 1, wherein the methanolic solution contains a ketone.

3. A process as claimed in claim 1, wherein the methanolic solution contains from 5 to 20% by weight of methylethyl ketone or cyclohexanone.

4. A process as claimed in claim 1, wherein current densities of from 5 to 50 amps/dm. are used.

5. A process as claimed in claim 1 wherein the electrolytic cell is intermittently stopped for periods of from 5 to 10 seconds at intervals of from 2 to 20 minutes.

References Cited UNITED STATES PATENTS 3,652,430 3/1972 Beck et al 20459 R FOREIGN PATENTS 880,289 6/1953 Germany 20459 R 1,181,688 11/1964 Germany 20459 R 154,531 1/1962 Russia 20459 R FREDERICK C. EDMUNDSON, Primary Examiner U.S. Cl. X.R. 20472 

